As a use of renewable energy rapidly increases, a need for an energy storage device using a battery rapidly increases. As the battery, a lead battery, a nickel/hydrogen battery, a vanadium battery and a lithium battery may be used. However, the lead battery and the nickel/hydrogen battery have significantly low energy density which is a problem in that a large space is required to store energy having the same capacity. In addition, the vanadium battery has a problem in that environmental pollution is caused by using a solution containing heavy metals, and amounts of anode and cathode materials that are moved through a membrane separating the anode and the cathode are small, which causes deterioration of performance, such that mass-production of the vanadium battery has not been achieved yet. The lithium battery having significantly excellent energy density and output characteristic is technically advantageous. However, it is difficult to be economically used as a secondary battery for large scale power storage due to scarcity of a resource of a lithium material.
To solve this problem, there are a number of attempts to use sodium, which is an abundant resource on the earth, as a material of the secondary battery.
Among them, as described in US Patent Application Publication No. 20030054255, a sodium-sulfur battery using beta alumina having selective conductivity with regard to sodium ions and having an anode supported with sodium and a cathode supported with sulfur, has been currently used as a large scale power storage device.
However, upon considering conductivity and melting point of battery components, the existing sodium-based secondary batteries such as a sodium-sulfur battery or a sodium-nickel chloride battery have disadvantages. Specifically, the sodium-nickel chloride battery has an operating temperature of at least 250° C., and the sodium-sulfur battery has an operating temperature of at least 300° C. Due to the above-described problems, the sodium-based secondary batteries are economically disadvantageous in view of manufacture or operation for maintaining temperature, for maintaining air tightness, and for reinforcing safety.
In order to solve the foregoing problems, a room temperature-type, sodium-based battery is being developed, however, which has a very small output, and is much less capable than a nickel-hydrogen battery or a lithium battery.